1. Field of the Invention
The present invention relates to fiber optic transmission systems in general and in particular to a transceiver for providing full duplex signalling in an optical transmission system comprising a single fiber optic cable.
2. Description of the Prior Art
A full duplex communication transmission system in general comprises one or more transmission line(s) having both a receiver and a transmitter, i.e. transceiver, located at each end of the line(s) for simultaneously receiving and transmitting signals. Whatever the nature of the signals being transmitted and received by each of the transceivers, when a single transmission line is used some method or means must be provided for separating the signals, i.e. for distinguishing the transmitted signals from the received signals.
In known systems for transmitting and receiving electrical signals on a single coaxial cable, also known as an electrical hybrid, full duplex operation is achieved using differential amplifiers at each end of the cable. In a system of this type, for example, one half of the transmitted power is applied by means of a pair of voltage dividers to each input of the differential amplifier at each end of the cable and to one end of the cable. Since the same signal, i.e. the transmitted signal, is applied to both inputs of the differential amplifier, there is no output resulting therefrom. However, a signal received from the cable being applied to only one of the amplifier inputs will unbalance the amplifier and produce a corresponding signal on the output thereof. Thus, simultaneously transmitted and received signals are separated.
In optical transmission systems, previous proposals for achieving full duplex communication over a single fiber optic cable have involved suggestions for using different color optical sources in a frequency division multiplex scheme or using different polarizations of light (horizontal and vertical) to effectively separate each end of the duplex. The use of differential amplifiers or other types of comparators and associated circuits for providing full duplex operation in an optical transmission system does not appear to have been disclosed or suggested.